(a) Field of the invention:
The present invention relates to a wide-angle zoom lens system.
(B) Description of the Prior Art:
A zoom lens system comprising a negative lens group and positive lens group for which a mechanical compensating method is adopted is known already. For this kind of retrofocus-type zoom lens systems, it is possible to make the field angle larger compared with the widely known zoom lens systems comprising a lens group constituting an afocal optical system. Moreover, it is possible to make the back focal length large even when the field angle is made large. When, however, it is attempted to make the aperture ratio of the lens system especially large, the effective N.A. of the rear positive lens group in the wide-angle position, where the field angle of the lens system is the largest, becomes considerably different from the effective N.A. in the tele-position, where the field angle of the lens system is the smallest.
Therefore, it becomes very difficult to favourably correct variation of aberrations at the time of zooming. This is the same also in case that it is attempted to make the zooming ratio larger. Besides, for the front negative lens group, aberrations should be kept stable when the entrance pupil moves. For the rear positive lens group, spherical aberration should be kept stable when N.A. changes.